Heat-treated monocalcium phosphate



Patented May 30, 1939 UNITED STATES 3.160.883 nan-Tasman uonocswmm PHOBPHATE- .Iulian m schlaeger, Chicago Heights, 111., asslgnor to VictorChemical Works, a corporation of Illinois No Drawing.

Application August 22, 1030,

Serial No. 228-174 19 Claims.

This invention relates to an improved baking acid and more particularlyto an anhydrous monocalcium phosphate baking acid having a slow rate ofreaction in dough mixtures. I

The baking characteristics of ordinary monocalcium phosphate monohydrateare well known, and the material has been used extensively in commercialbaking powders, seli rlsing flour. and in ready mixed cake, walls anddoughnut flours. The material has one serious disadvantags in that itsprimary rate of reaction with the sodium bicarbonate commonly employedin dough mixtures is too high for the most emcient utilization of itstheoretical leavening power. correspondingly the rate of reaction in thesecondary period is low. Heretofore attempts to slow down its primaryrate of reaction have been ineffective.

The term primary reaction rate means the amount of CO: generated in thefirst two minutes by reaction of the material in a water medium withsodium bicarbonate at 27 C. with a standard quantity of bicarbonatecapable of liberating 200 c.c. of C02. The secondary reaction rate isthe amount of C02 generated in the succeeding 8 minutes. In some casesthe rates are measured on dough mixes instead of water, and in suchcases the primary period is two minutes, but the secondary period is 13minutes instead of B. In the present description the water rate isreferred to in all the specific examples unless the dough mix isspecifically referred to. The percentage evolved in either period may beobtained by dividing the rate by two.

Sodium acid pyrophosphate has been employed as'a baking acid and, whileit reacts more slowly, leaves'a slightly bitter, astringent taste in thebaked product, which is known as "pyro flavor. This flavor is seriouslyobjected to by many bakers. On this account any attempt to retard therate of reaction of monocalcium phosphate should avoid the production ofpyrophosphate in an amount suflicient to produce this "pyro flavor".

In accordance with this invention a monocalcium phosphate having amarkedly lessened primary reaction rate and an increased secondaryreaction rate is produced without the production of pyrophosphate insufliclent quantities to produce pyro flavor". In accordance with thepreferred form of this invention, anhydrous solid crystallinemonocalcium phosphate is heated under carefully controlled conditions toproduce what appears as a thin skin coating, which is very slowlyPenetrated by water and which protects the interior of the particleduring the initial stages of the preparation of the dough.

Grinding of the material after heat treatment restores substantially theoriginal rate of rea'ction presumably by destroying the covering sheetof the coating. It is, therefore, important that the starting materialhave the necessary fineness of division desirable in a baking acid.

Similar results may be accomplished by heating porous anhydrousmonocalcium phosphate to improve that product, but the resultantmaterial is less altered in its rate of reaction than that produced byheating the solid non-porous material and does not retain its alteredreaction rate so well upon standing in a moist atmosphere.

The heat treatment comprises subjecting the anhydrous monocalciumphosphate particle to a temperature above 140 C. for a period of timesuflicient to decrease its initial reaction rate with sodium bicarbonateat least 10% and preferably at least 35-45% and to increase itssecondary reaction rate correspondingly. This should be carried outwithout converting any appreciable amount 01' the orthophosphate topyrcphosphate. The exact chemical changes taking place are not fullyunderstood. It is possible that a thin skin of insoluble metaphosphateis formed on the surface of the product. Photomicrographs indicateclearly the formation of a shell upon solution of the soluble phosphatein which segregated particles of dicalcium phosphate are held.

Whatever the chemical changes may be, physically a thin, transparent,fused or glass-like skin appears around the particle, which isrelatively insoluble as compared to the interior. When the product isemployed in a baking preparation, this skin delays the reaction with thesodium bicarbonate oi the baking preparation and permits the preparationoi wet dough and batter mixes without large losses of the leaveninggases. As a result, a larger proportion of gas is liberated in the earlybaking stage in the oven.

The preferred anhydrous monocalcium phosphate is prepared as describedin the copending application of William H. Knox, Jr.. Ser. No. 226,180,filed August 22, 1938. A typical example of that procedure is asfollows:

A quantity of 57.5 Baum gravity strength blast furnace phosphoric acidwas heated to approximately 110" (2., placed in a hatch mixer equippedwith an efiicient agitator. and quicklime equivalent to about 70 to 80%of that theoretically required to produce monocalcium phosphate wasadded as rapidly as possible without permitting the reaction temperatureto rise above 170 C. The lime had previously been ground to at least 100mesh in orderto increase the rapidity of the reaction as well as thedistribution thereof. As the temperature began to drop below 165 0., dueto the evolution of steam, small additions of hydrated lime were made,causing the temperature again to rise. As the temperature approached1'70 0., water in small amounts was sprayed into the mixture, but thetemperature was not permitted to fall below C. The reaction temperaturewas thus maintained until suiliclent hydrated lime had been added toobtain a product which was definitely neutral to methyl orange.Approximately 2.5% excess lime over that theoretically required was usedin the batch. Under the conditions outlined, the batch temperature wasmaintained between C. and 160 C. for 25 minutes.

This material is then passed through a screen, the openings of which areat least as small as 100 mesh and preferably of 200 mesh size. Theproduct will generally have a neutralizing value of about 84 to 93, andpreferably 87 to 91, in terms of parts by weight of sodium bicarbonatecompletely neutralized by 100 parts by weight of the acid phosphate. Thematerial is an eilective baking acid without further treatment and isclaimed as such in the copending application of William H. Knox, Jr.,Serial No. 226,180, filed August 22, 1938. The neutralimng value of theproduct falls approximately 3 to 4 points on heat treatment, so thatwith a range of 87-91 originally the heat treated material will be fromabout 83-88.

The baking characteristics of this product, however, are greatlyimproved by further heat treatment, which is efl'ected by heating theparticles to a temperature above 140 C. and below a temperature at whichsubstantial quantities of pyrophosphate are formed. This treatment maybe made by passing the material through a rotary kiln, a shelf drier, oreven by dropping the material through a flame or heated chamber. Thehigher the temperature employed, the shorter will be the time requiredto effect the formation of the skin-like coating over the particles.Where flame temperatures are employed the time of contact is notsufllcient for a thorough heating of the particle.

Temperatures of the material about 230 C. tend to cause rapid conversionof the orthophosphate to pyrophosphate and should be avoided.

Temperatures between 190 C. and 230 C. likewise tend to cause theformation of pyrophosphate unless precautions are taken, particularlywith respect to the amount of moisture present and the degree ofneutrality.

Where the starting materials are completely anhydrous at the beginningof the operation (1. e. do not become anhydrous during the operation) itis preferred to employ a heat treating temperature of 200 C. to 220 C.The lower temperatures produce a heat treating effect but requirerelatively long periods of exposure. For example, at 140.C., from 5 to20 hours are necessary, depending upon the degree of improvementdesired.

For best results the heat treatment is normally carried forward to apoint just under that at which substantial amounts of pyrophosphatebegin to form. The production of more than 10% of pyrophosphate in thematerial produces a "pyro taste" in the product. However, the presenceof smaller amounts of pyrophosphate has a decided effect upon thestability of the prodnot under relatively humid conditions. For example,a product having a neutralizing value of 01.0 and containing 3.4%pyrophosphate had a secondary reaction rate of 117 c. c., which droppedto 45 c. c. after 21 days in an atmosphere of 65% relative humidity at39 C. On the other hand, a similar product having a neutraliza,1eo,ass

ing value of 81.2 with a 2.5% pyrophosphate had a secondary reactionrate of 119 c. c. when made and this had dropped only to 72 c. c. after21 days under the same storage conditions. While both of these productshave considerable commercial value, the second is obviously muchsuperior. It is therefore preferred, from the standpoint of stability ina moist atmosphere, to keep the pyrophosphate below 5% and preferablybelow 3%.

In the heat treatment it is preferred to approach the maximumtemperature gradually, particularly at temperatures about C. It ispossible that this procedure insures the removal of all traces ofmoisture before a pyrophosphateforming temperature is reached.

In heat treating a hydrated monocalcium phosphate the temperature ismaintained at 100 C. to 180' C. until all crystal water has been removedand the product then heated at 180 C. to 220 C. until the desiredimprovement has occurred.

With a properly prepared starting material of the Knox type, thetemperature and time may be varied over a considerable range withoutgreatly affecting the character of the product. Good results have beenobtained by heating such material for one hour at 225 C. or severalhours at 220 C. In preferred practice, the material is passed through arotary converter countercurrent to a flow of heated air at such ratethat the material discharges at a temperature of 210- 220 C. Under theconditions used, the material is in the converter for about 2 hours andis at a temperature above 200 C. for approximately 30 minutes.

While baking acids are customarily stored in sealed containers, they arefrequently left in an imperfectly sealed condition on shelves for longperiods. For greatest commercial value, therefore, maximum stabilityagainst rehydration is desirable in an anhydrous product such as thatherein described.

In order that the heat treated product have such greatest possiblestability toward moisture on storage, considerable care should beobserved in selecting it. The heat treatment may be applied to anyanhydrous monocalcium phosphate, whether solid or porous, and whethercrystallized in a substantially pure form, or whether made anhydrous bydehydration of hydrated monocalcium phosphate. Materials other than thesolid, non-porous crystalline material prepared by the Knox process arerelatively unstable, however. For example, an anhydrous materialprepared by crystallization in a substantially pure state from strongphosphoric acid solution after heat treatment had a secondary reactionrate of 83 c. c. in a dough mixtue, which dropped to 26 c. c. after 5days storage at 35 C. and 65% relative humidity.

Heat treatment of anhydrous material prepared by first heatingmonohydrated monocai cium phosphate to drive off the water ofcrystallization produces diilerent results both in the amount by whichthe reaction rate may be altered and in the stability of the product inthe presence of water vapor. These diflerences are due to such factorsas physical condition of the material, the batch conditions underwhichit was made, the degree of neutrality, and the presence the methodof its preparation has a decided effect upon the heat-treated material.

Theconditions under which the reaction for producing the anhydrousmaterial is carried out should he, therefore, for maximum stability,very carefully controlled. In this connection, the reaction temperatureshould at all times be in excess of 140 C. to avoid formation ofhydrated material, and the temperature should be controlled and producedby the heat of reaction of the materials. The phosphoric acid used ispreferably heated in advance and its concentration is made great enoughthat the reaction will generate suflleient heat to elevate thetemperature of the materials as well as the water in the phosphoric acidsolution to the requisite temperature.

Under normal conditions this requires a phosphoric acid having a Baumgravity strength in excess of 54. With phosphoric acids near this borderline, it is preferable to use only quicklime as the other reagent, butwith acids of higher strength the lime may be partially or entirelyhydrated. In any event, the amount of water present, either with thephosphoric acid or with the lime, is made low enough notto reduce thereaction temperature below 140 C. It is preferred not to use an acidhaving Baum gravity strength above 65, because of the mechanicaldifficulties in securing rapid and intimate mixing in the reaction insuch case.

It is preferred that in preparing the anhydrous monooalcium phosphatethe total water present at the beginning of the reaction, including thatproduced by the reaction, be not less than 15% and not more than 32%based on the total CaO and HaPOe and that the elimination of this waterfrom the reaction mass be accomplished under such conditions that a dryproduct results without any substantial formation of a hydratedmonocaloium phosphate.

In the preferred preparation process, the temperature of the reaction iscarried considerably above 140 C., best results having been obtained inthe range from 160 C. to 170 C. or sometimes 175 C. Where the upperlimit is employed, greater care must be exercised to avoid the formationof pyrophosphate. The presence of any considerable amount ofpyrophosphate distinctly lessens the value of the product, both bylessening its stability and by aflecting its ability to be heattreatedunder the present process. The process should, therefore, not beoperated above 175 C. for any extended period of time.

When the process is operated at a reaction temperature of 160 C. to 170C. it is preferred to correlate the strength of the acid, the degree ofhydration of the lime, and the original temperature of the reactingingredients, so that they are capable of raising the temperature above170 C. Then by rapid mixture of lime and acid, the temperature isbrought quickly to at least 160 C. and is maintained within the rangeduring subsequent additions of lime by the use of a volatile liquid,such as water, as a cooling agent. This is accomplished by sprayingsmall quantities of water from time to time directly into the reactingmass, the latent heat of the water and the dispersion of it serving toprevent any local overheating which would not be prevented by indirectcooling. Particularly where water is the material sprayed, it ispreferable not to add it after the last addition of lime.

The amount of lime employed should be suchastoproduceaneutralisingvalueintheflnal product below and generallyabove 50, amount preferably being such as tralising value in the rangeof 84 erably 8'! to 91, the product of this range having greaterstability, other conditions being equal, against rehydration.

Itis alsopreferred thatthe-reaotioninthe higher temperature range becarried out as rapidly as possible, inasmuch as the formation ofpyrophosphate, or the tendency to form wronhosoimeisgreatlyincreasedbytheincreaseofthetime of reaction. When in the mannerabove set forth, by which the reaction temperature is rapidly raised toC., the reaetionmay be completed in 20 to 25 minutes. It is preferredthat it should be less than 30 minutes. Naturally, longer times arenecessary in the lower ranges of the temperature, and at C. the time ofreaction is considerably longer.

The acid employed may be any reasonably pure acid. but it is preferredthat the acid be blast furnace acid or have the impuritiescharacteristic of such blast furnace acid. The inclusion of theseimpurities in the product of this invention is valuable in fitting itfor heat treatment in accordance with the present process.

In another example, where the batch temperature was maintained betweenC. and C. for 25 minutes, the product, after heat treatment inaccordance with the present process, had an initial two to ten minutebaking powder rate of 119 c. c. which. after 21 days at a temperature of39 C. and a humidity of 65%, had decreased to 72 c. c.

As stated before. it is preferred to add a slight excess of lime. 'lheneutralizing value of pure anhydrous monocalcium phosphate is 95.7, thisterm meaning the parts by weight of sodium bicarbonate, which will beutilized by 100 parts by weight of the acid phosphate according to thefollowing type reaction:

The excess of lime here added eliminates any free acid from the product.Some of the excess lime appears in the final product as dicalciumphosphate, which appears to be concentrated in spots on the surfaces ofthe crystals. A small amount of free lime may be present in the productwithout materially aflecting the baking characteristics.

The method used for testing neutralizing values is that published by theAmerican Association of Cereal Chemists as method 5b on page 117 ofCereal Laboratory Methods. 3rd edition, 1935.

The stability of the product under storage in a moist atmosphere isconsiderably affected by the presence of impurities. Pure material doesnot retain its activity well in a damp place. The precise inter-relationof the impurities, however, has not been determined. Impurities whichare characteristic of blast furnace phosphoric acid and ordinary highgrade lime appear to produce substantially the best results.

The protective coating or medium appear to include an insoluble alkalimetal metaphosphate. possibly in combination with some calcium compound.Analysis of the residue of heat treated materials remaining aftersolution of the remainder in water showed conversion of veryconsiderable amounts of alkali metal material from soluble to insolubleform during heat treatment.

The eifect of these impurities is particularly noticeable in connectionwith the solid, nonporous, crystalline type of product made by the Knoxprocess. For example, a product crystallized from a strong purephosphoric acid solution, after heat treatment, completely hydrated inless than 48 hours at 39 C. in an atmosphere of 65% relative humidity.On the other hand, a heat treated product made with blast furnace acidcontaining its normal impurities showed an original secondary reactionrate of 123 c. c. and even after 10 days storage in a humldor at 39 C.and 65% relative humidity, still showed a secondary rate of 100 c..c. Atthe end of 21 days under the said conditions the secondary rate wasstill 93a. 0. This product showed the following impurities:

Per cent Acid insoluble 0.09 FePOr 0.15 AIPO; 0.26 MgO 0.19 S01 0.21 K200.38 mm 0.17 The addition 01 impurities to pure phosphoric acidsuillcient to give this final composition in the anhydrous monccalciumphosphate will make it' as suitable as the blast furnace acid itself.

01 the impurities noted, not all seem to be requisite in producing theimproved result, the

alkali metals being the most eifective.

The function of the impurities is not chemically understood but itappears that during the batch crystallization the impurities concentrateon the surface of the minute crystals and then on heat treatment areconverted to insoluble phosphates, possibly fused or partially fused,which form a skin coating over the monocalcium phosphate. It is possiblethat the alkali metals serve merely as fluxing agents which lower themelting point of this coating. However, investigation has indicated thatthe percentage of insoluble iron and aluminum compounds does not altersubstantially upon heat treatment, whereas the amount of insolublepotassium and sodium compounds increases enormously. For example, in oneinstance, an untreated sample showed only 0.01% K20 insoluble, but afterheat treatment showed 0.17%. In the same sample the N320 insolublebefore heat treatment was 0.017%. which increased to 0.078%v on heattreatment.

At the same time there are indications that a small amount of thecalcium is converted to insoluble phosphate.

The following tables indicate the eflfect of the addition of variousimpurities. Table I shows five batches of heat-treated anhydrousmonocalcium phosphate, the first batch being made from substantiallypure acid and the remainder made from pure acid to which the indicatedimpurities had been added. It will be noted that M appears in all casesdue to its presence in the lime employed:

Table II shows the original secondary reaction rate of each batch andthe secondary reaction after 1, 3 and days storage at 39 C. and 65%humidity.

Table '1! After 1 Alter a After 10 Batch mun day days am e t. 6.6. c C.c C.

102 10 w w 121 m 100 as 123 104 sa 124 m m 81 123 m 104 91 In otherexamples commercially pure phosphoric acid was employed, to which onlysodium and potassium were added in accordance with the following table:

Table III The product was stored for the three days in a humidor at 39C. and 65% relative humidity.

The product containing 0.9% of K20 was, however, not as stable as thosecontaining smaller amounts. It is preferred to employ only 0.1 to 0.5%of alkali metal impurities, although amounts up to 1% are quitepracticable. For example, anhydrous monocalcium phosphate was preparedwith blast furnace acid to which 0.75% K20 had been added. 2,000 lbs. ofthe above acid at 57 Baum gravity strength was heated at 110 C. and 350lbs. of finely ground quicklime quickly added to the batch mixture. Thetemperature of the mixture rose quickly to 160 C., and further additionsof lime were made at such rate that the temperature remained within therange of 160 C. to 173 C. until the mixture was definitely neutral'tomethyl orange indicator. The product was then heat-treated at atemperature of 190 C. to 230 C. The product showed a pyrophosphatecontent of between 3% and 4% and a secondary reaction rate of 120 to 129c. c. After four days storage in a humidor at 39 C. 65% relativehumidity, the secondary reaction rate was 93 c. c. and after nine dayswas '14 c. c.

A typical lime employed had the following composition:

Table IV Per cent CaO 98.

MgO .3 Insoluble Mostly silica .45 FecO: and A120: .2 S0: .03 Loss onignition 0.9 K20 0.007 NazO 0.028

(Balance undetermined) Blast furnace acid as here discussed is acidwhich has been produced in the customary manner, the productionincluding the removal of undesirable or toxic impurities such as lead.arsenic and fluorine. A typical approximate composition for 56 Baum acidis as follows:

Table V Per cent P205 56.6 H3PO4 78.8 FePO; 0.1-0.2 AlPOr 0.2-0.3 CaO.02-.03 MgO .02.03 K20 0.20-.40 NazO 0.1-0.2 S102 .01 S: 0.2-0.3 Mn, As,F, Ni. etc less than 0.05

For the preferred product it is desirable that the product shouldcontain at least 0.1% of an alkali metal, particularly potassium. Itshould not contain after heat-treatment more than of material classifiedas pyrophosphate" and should after heat-treatment have less than 3% ofsuch material. It should have the property of reacting with sodiumbicarbonate in water solution or wet dough mixtures at such rate thatless than 25% or the reaction is completed in the first two minutes(primary reaction rate), while at least 35% of the total gas evolutiontakes place within the succeeding two to ten or two to fifteen minuteperiod (secondary reaction rate). The stability of the product should besuch that after 20 days storage at 39 C. in an atmosphere of 65%relative humidity, the secondary reaction rate is still at least 60 c.c., or 30% of the total theoretical gas. The product should besubstantially less than 100 mesh particle size and preferably less than200 mesh, and should not contain any free acid, and should have aneutralizing value below 95 and generally over 80, preferably from 83 to88. A product carefully made from blast furnace acid, or one having theimpurities characteristic thereof in this respect, reacted with limeunder the preferred conditions outlined, and then heat-treated as hereindicated, will have a primary reaction rate of less than 20% and asecondary reaction rate of at least 50%, and the latter will remain ashigh as 35% or more after 20 days storage at 39 C. and at 65% relativehimidity.

The stability of the heat-treated product against hydration in a moistatmosphere was tested by placing a sample in a humldor at 90 F. in anatmosphere of 60% relative humidity, and determining the gain in weightafter definite periods of time. Complete hydration is when the producthas absorbed suflicient water to form the monohydrated salt. Thefollowing table 11- lustrates typical comparative hydrationcharacteristics of pure crystalline anhydrous monocalcium phosphate(Pure A. M. C. P.); and the heat-treated product of the presentapplication (Heat-treated A. M. C. P.).

Table VI Percentage of theoretical hydration Number of days eat-tr atedM. C.

As hereinafter stated, however, it is possible also to improvethesecondary reaction rates of anhydrous monocalciumphosphates,whicharemuch less adapted to heat-treatment than thepreferred base materials. For example, Table VII gives an analysis ofthree hydrated monocalcium phosphates which were heattreated accordingto the present invention by first dehydrating and then heat-treating.

The product A showed a secondary dough reaction rate of 73 c. c. afterheat-treatment, as compared with an original secondary rate of 6 c. c.before dehydration.

The product B showed a secondary rate 01' 24 c. c. after heat-treatmentas compared to an original one of 13.

Product C increased its secondary reaction rate from 15 to 30 c. c. onheat-treatment.

Table VII Neutralizing value 83.8 83. 4 81. 4 Free acid percent. 0 0 0.4 Acid insoluble. do 0. 05 0. l2 0. 04 Fe and AlP0 do 0. 42 0.08 0.56FePOa d0-- 0.19 0.035 0.14 Total P101; 00.. 55.5 55.5 55.5 W. 8. do51.55 52. 2 5i. 6 80; d0 0.21 0.08 0. 24 080-- do 23.18 23.0 21.8 MgO do0. 18 0.16 0.54 MIL do.. 0. 003B 0. 0003 0.10 Pia. p. m l. 3 5 2 p. p.m. 4.0 12 36 A810: p. p. m 0. 5 0. 2 0. 3 H (iree) rcent 0. 38 0.08 0.35 Loss Onignitin do 21.0 21.0

:0 do 0. 22 0.037 0.16 N810 do 0. 50 0.038 1. 33

A small amoum 0! free acid may be formed on the surface of the lessalkaline material during the heat treatment, which renders the particlesslightly sticky. To overcome this it is preferred to add a small amountof a neutralizing agent in powdered form, for example, by addingprecipitated chalk, hydrated lime, tricalciurmphosphate or magnesiumcarbonate. Usually from to 2% of this conditioning agent is sufficientto render the product entirely free-flowing and also con tributes to itsstability against hydration during storage in humid climates.

When heating the porous anhydrous monocalcium phosphate, the skinapparently forms not only on the outer surface of the particle butappears to follow the pores of the material.

It appears, however, that with this material prepared by heatinghydrated monocalcium phosphate at a temperature and period of timesufliclent to produce a skin coating an appreciable amount of theorthophosphate is converted to a more molecularly dehydrated form.

For example, regular hydrated monocalclum phosphate of 200 mesh size washeated over 1% hours to 230 C. At the end of this time about 7% of thematerial had been converted to pyrophosphate, and the particles had askin coating which slowed down the reaction rate to the extent of 27% inthe first two minutes as compared to 67% for the initial material. Thematerial also showed 34% of reaction in the succeeding 13- minuteinterval.

The slower reaction rates of the heat-treated materials in the aboveexamples are extremely advantageous in the baking of cakes, biscuits,etc. For example, the slow evolution of CO: gas during the first twominutes of the dough mixing stage permits a thorough mixing of the doughor batter without an excessive loss of leavening gas during this period.In other words, a high percentage of the leavening capacity of thebaking powder is reserved for the baking stage. Greater specific volumeof the baked product is obtained. For example, in the case of biscuitsthe relative specific volumes obtained when using the heattreatedmaterial are as follows: using ordinary hydrated monocalcium phosphateas the basis of comparison, the product obtained from heating the solidanhydrous monocalcium phosphate had a specific volume about 25 to 35%greater, and the product described in the preceding paragraph had aspecific volume about 20% greater.

Another important advantage in the use of heat-treated products is thefact that the baked products do not have the characteristic slightlybitter, astringent taste normally associated with the use ofpyrophosphate baking acids such as sodium acid pyrophosphate. It ispermissible in the heat-treatment to form a small amount ofpyrophosphate. Such pyrophosphate would, of course, contribute itsproportion of "pyro flavor", but this flavor is not noticeable in thebaked product until the pyrophosphate content of the baking acid ispresent in an amount greater than one-fourth of the total baking acid.In this process the heating can be easily controlled to prevent theformation of more than 10% of a molecularly dehydrated phosphate even inthe case of the hydrated monocalcium phosphate and still obtain asatisfactory skin coating of the particles. In the case of the speciallyprepared solid non-porous crystalline material I find in no case is itnecessary to heat sufiiciently long to convert more than of theorthophosphatc to a more highly dehydrated form.

Microscopic observations show that when the heat-treated crystals areplaced in water that solution is very slow and that solution beginswherever there is a break in the skin coating, the interior of theparticles dissolving first, leaving a thin transparent shell 01'relatively insoluble material. The relative imperviousness oi the outershell or skin coating appears to control the rate of solution 01' thecore 01' anhydrous monocalcium phosphate.

The relative stability and slow reaction rate oi the new heat-treatedanhydrous monocalcium phosphate permits for the first time the use ofsmall particle size calcium acid phosphate in commercial baking powders.Heretoiore the commercial phosphate baking powders have been composed ofgranular materials, the greater portion of which was larger than 200mesh size and smaller than 140 mesh in size.

The new product may be used in particle sizes smaller than 140 mesh oreven less than 200 mesh in admixture with sodium bicarbonate to give acommercial baking powder of good keeping quality during storage. Theprior difllculty was to choose a gradation of particle sizes oimonocalcium phosphate monohydrate for baking powders which are not solarge that black specks result on the surface crust oi the baked productand not so small as to cause premature liberation of the leavening gasduring the storage of the baking powder. As a result, the commercialacid phosphate baking powders have been made from a granular calciumacid phosphate ranging from about 120 mesh to not less than 200 meshsize particles. The present product on the other hand is preferably usedin particle sizes substantially most of which are smaller than 200 mesh.

The slow reaction rate and stability against hydration of this productmakes its use highly advantageous in self-rising flours. The new productis especially applicable to cake and waille readymade flour mixtures, inthat such mixtures can be made to include higher ratios of sugar andflavoring ingredients, thus permitting the making of more tasty bakedproducts. Normally the ready-mixed cake and waiile flours on the marketcontain sodium acid pyro-phosphate because of its slow reaction rate,but the baked products are not entirely satisfactory. The characteristic"pyro flavor" is present, and, when the acid is used at its correctneutralizing value the pyrophosphate and products give the baked productan alkalinity oi the order oi a pH value or '1.8. This degree ofalkalinity has a detrimental effect on the flavoring ingredients. 0n theother hand, it the sodium acid pyrophosphate is used at suchneutralizing value as to insure a lower pH value of around 1.0 theintensity of the pyro flavor is increased. When substituting the presentheat treated product for the sodium acid pyrophosphate in the preparedcake and waiile flours, it can be used at its correct neutralizingvalue, giving a baking product with a pH value of the order oi 7.0 to1.1 without any "pyro flavor. The lower alkalinity will permit a betterdevelopment and retention of the desirable flavors without the use ofexcess amounts of flavoring ingredients to offset a "pyro flavor". Thusa more desirable ready mixed cake, biscuit, or waiiie flour can beplaced on the market! The prepared self-rising flour mixes may includethe proper amounts of sugar, shortening, salt and other flavoring agentsto give the desired type of baked product.

The emciency o! the heat-treated baking acid in the baking stage is sogreat that the amounts of leavening ingredients may be materiallyreduced while still obtaining a superior baked product from thestandpoint 01' volume, texture and flavor. This is true, either in thecase or sell! rising flours or home or bakery mixed doughs. For example,biscuits baked with two-thirds of the amount of soda and baking acidnormally required when hydrated monocalcium phosphate is the baking acidstill have a greater specific volume and superior texture.

For example, Table VIII shows the specific volumes of biscuits whenprepared from dough made with ordinary monocalcium phosphate (M. C. P.)as compared with those irom the present heattreated anhydrousmonocalcium phosphate (A. M. C. P.)

Since one-third more volume is obtainable in biscuits where the dough isnot permitted to stand for excessive periods, it is possible to use atleast one-fourth less of the leavening constituents to produce biscuitsof normal size.

In self-rising flours it is customary to employ 1 baking soda (based onthe flour) together with the equivalent amount of baking acid. With thepresent material this may be decreased to 1.0% and still producebiscuits of superior volume as compared to the former mixes containingordinary monocalcium phosphate.

For example, in a self-rising flour containing 1.0 part sodiumbicarbonate, 1.75 parts salt, 1.18 parts heat-treated anhydrousmonocalcium phosphate and parts flour gave a superior volume when bakedas compared to biscuits made from a flour containing 1.5 parts sodiumbicarbonate, 1.875 parts ordinary hydrated monocalcium phosphate, 1.75parts salt and 100 parts flour, mixing and baking conditions beingidentical.

The character of the baked article is quite different from that ofordinary biscuits, both with self-rising flour and freshly mixed doughs.The texture resembles closely that of yeast made bread, the cells arethin-walled and large, the crumb has a flaky appearance and tenderfeel,and the side walls are smooth. This improvement is apparently dueto the slowly soluble nature of the new heat-treated baking acid, whichpermits the soda to dissolve first and thus cause a preliminarydevelopment of a suitable gluten condition in an alkaline medium beforethe neutral condition develops.

Where pyrophosphate contents are given they were determined bydissolving a sample of the product in dilute hydrochloric acid,neutralizing with N/10 caustic soda to a brom phenol blue end point,then adding an excess of zinc sulfate to precipitate the pyrophosphateas a zinc salt and titrating the liberated sulfuric acid with N/10 NaOHto a brom phenol blue end point and calculating the result as calciumacid pyrophosphate.

The term non-porous" as used in this specification and in the claims isintended to mean a solid particle not built up from an agglomeration ofminute crystals with intervening fissures and spaces between thecomponent parts of the whole particle nor a particle resulting from thedriving out of water by heat from a crystal which contained water ofcrystallization.

The term "autogenous coating as used in the claims hereof means that thecoating is formed from an ingredient or ingredients of the crystalitself by a process including heat. The coating may of course includeingredients which are not natural to the crystal The terms 100 mesh and200 mesh sizes are used in this specification and claims to meanparticles which have been passed through standard 100 mesh and 200 meshsieves having openings of 0.0058" and 0.0029", respectively.

This application is a continuation in part of my copending applicationSerial No. 149,025, filed June 18, 1937.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, but the appended claims should be construed as broadly aspermissible, in view of the prior art.

I claim:

1. As an article of manufacture, particles of finely divided anhydrousmonocalcium phosphate having a thin, autogenous, glassy, substantiallycomplete, relatively insoluble coating, the particles of saidmonocalcium phosphate being substantially free from pyrophosphate.

2. As an article of manufacture, particles of finely divided anhydrousmonocalcium phosphate having a thin, autogenous, vitreous. spbstantiallycomplete, relatively insoluble coating, the particles of saidmonocalcium phosphate containing less than 10% of pyrophosphate.

3. Anhydrous monocalcium phosphate as set forth in claim 23 in which thematerial has a primary reaction rate with sodium bicarbonate of not morethan 25% and a secondary reaction rate therewith of not less than 25%.

4. Anhydrous monocalcium phosphate as set forth in claim 23 in which theparticles contain from .1% to 1.0% of alkali metal calculated as oxide.

5. Anhydrous monocalcium phosphate as set forth in claim 23 in which theparticles contain less than 3% of pyrophosphate.

6. As an article of commerce, particles of finely divided non-porousanhydrous monocalcium phosphate substantially pyrophosphate-free havinga thin, autogenous, glassy, relatively insoluble coating.

7. As an article of commerce, particles of finely divided non-porousanhydrous monocalcium phosphate substantially pyrophosphate-free havinga thin, autogenous, glassy. substantially complete, relatively insolublecoating.

8. Anhydrous monocalcium phosphate as set forth in claim 7 in which thematerial has a neutralizing value between 80 and 88.

9. Anhydrous monocalcium phosphate as set forth in claim 7 in which theparticles of anhydrous monocalcium phosphate contain impuritiescharacteristic of blast furnace phosphoric acid.

10. A product as set forth in claim 7 in which. the anhydrousmonocalcium phosphate has been prepared in solid crvstalline form bymixing blast furnace phosphoric acid and lime at a reaction temperaturesubstantially in the range of C. to C. and maintained within that rangeuntil completion of the reaction.

11. As an article of commerce, particles of finely divided non-porousanhydrous monocalcium phosphate having a thin, autcgenous, glassy,vitreous, substantially complete, relatively insoluble coating, theparticles containing less than 3% of pyrophosphate.

12. As an article of commerce, particles of finely divided crystallinenon-porous anhydrous monocalcium phosphate having a thin, autogenous,glassy. substantially complete, relatively insoluble coating, saidparticles containing less than 10% of pyrophosphate and having areaction rate with sodium bicarbonate of not more than 25% and asecondary reaction rate of at least 35%.

13. As an article of commerce, particles of crystalline non-porousanhydrous monocalcium phosphate having a thin, autogenous, glassy,vitreous, substantially complete, relatively insoluble coating, saidparticles containing less than 10% of pyrophosphate and having a primaryreaction rate with sodium bicarbonate of not more than 20% and asecondary rate therewith of at least 50%, said product having astability against hydration such that after 20 days storage at 39 C. inan atmosphere of 65% relative humidity the secondary rate is still atleast 35%.

14. As an article of commerce, particles of finely divided non-porousanhydrous monocalcium phosphate substantially pyrophosphate-free havinga thin, autogenous, vitreous, substantially complete, relativelyinsoluble coating, said anhydrous monocalcium phosphate particles containing from .1% to 1.0% of alkali metal calculated as oxide.

15. A product as set forth in claim 14 in which the alkali metal is inlarge part potassium.

16. As an article of commerce, particles of I finely divided anhydrousnon-porous monocalcium phosphate having a thin, autogenous, vitreous,substantially complete, relatively insoluble coating, said particlescontaining less than 3% of pyrophosphate, having a neutralizing value of83 to 88, having a primary reaction rate with sodium bicarbonate of lessthan and a secondary rate of at least approximately the particlescontaining from .l% to 1.0% of alkali metal calculated as oxide, andhaving a stability such that after 20 days storage at 39 C. in anatmosphere of relative humidity the secondary rate is still at least35%.

17. Anhydrous monocalciurn phosphate having an integral vitreous,relatively insoluble skin coating thereon, containing less than 5%pyrophosphate, containing at least 0.1% alkali metal calculated asoxide, having a primary reaction rate with sodium bicarbonate of notmore than 25% and a secondary reaction rate of not less than 35%, andsuiliciently stable against hydration that at F. in an atmosphere 0! 60%relative humidity it will not become more than 5% hydrated in 5 days.

18. The method of producing a thin, vitreous, substantially complete,relatively insoluble coating upon finely divided particles of anhydrousmonocalcium phosphate, which comprises heating anhydrous monocalciumphosphate for a prolonged period at a temperature above C. and below atemperature at which substantial conversion to molecularly dehydratedphosphate occurs, to produce such thin, vitreous, substantiallycomplete, relatively insoluble coating without producing more than 5% ofpyrophosphate by such treatment, whereby the primary reaction rate 01'the anhydrous material with sodium bicarbonate is decreased to below35%, and the secondary reaction rate therewith is increased to at least25%.

19. The method as set forth in claim 18 in which the material is heatedto a temperature of 180 0., the temperature is then raised gradually to200 C. and is then maintained within the range of 200 C. to 220 C. for aperiod of approximately one-half hour.

JULIAN R. SCHLAEGER.

CERTIFICATE OF CORRECTION.

-Petent No. 2,160,252.

JULIAN R.

May 50, 1959.

SGHLAEGER It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows: Page 5, first column, line 1 9, for "himidity" read lmmidity;line 71 for the word "herematter" reed hereinberore; page 7 firstcolumn, line 1 8, after "crystal" insert a. period,- and second column,lines 2, 7, and 11, claims 5, i and 5 respectively, for the claimreference numeral "23" read 2; line 35, claim 10, for 0'." read 160? 0.;and that the said Letters Patent should be read with this correctiontherein that the same may conform to the record of the case in thePatent Office.

'Signed and sealed this 15th day of August, A. D. 1939.

(Seal) Leslie Frazer licting Commissioner of Patents.

finely divided anhydrous non-porous monocalcium phosphate having a thin,autogenous, vitreous, substantially complete, relatively insolublecoating, said particles containing less than 3% of pyrophosphate, havinga neutralizing value of 83 to 88, having a primary reaction rate withsodium bicarbonate of less than and a secondary rate of at leastapproximately the particles containing from .l% to 1.0% of alkali metalcalculated as oxide, and having a stability such that after 20 daysstorage at 39 C. in an atmosphere of relative humidity the secondaryrate is still at least 35%.

17. Anhydrous monocalciurn phosphate having an integral vitreous,relatively insoluble skin coating thereon, containing less than 5%pyrophosphate, containing at least 0.1% alkali metal calculated asoxide, having a primary reaction rate with sodium bicarbonate of notmore than 25% and a secondary reaction rate of not less than 35%, andsuiliciently stable against hydration that at F. in an atmosphere 0! 60%relative humidity it will not become more than 5% hydrated in 5 days.

18. The method of producing a thin, vitreous, substantially complete,relatively insoluble coating upon finely divided particles of anhydrousmonocalcium phosphate, which comprises heating anhydrous monocalciumphosphate for a prolonged period at a temperature above C. and below atemperature at which substantial conversion to molecularly dehydratedphosphate occurs, to produce such thin, vitreous, substantiallycomplete, relatively insoluble coating without producing more than 5% ofpyrophosphate by such treatment, whereby the primary reaction rate 01'the anhydrous material with sodium bicarbonate is decreased to below35%, and the secondary reaction rate therewith is increased to at least25%.

19. The method as set forth in claim 18 in which the material is heatedto a temperature of 180 0., the temperature is then raised gradually to200 C. and is then maintained within the range of 200 C. to 220 C. for aperiod of approximately one-half hour.

JULIAN R. SCHLAEGER.

CERTIFICATE OF CORRECTION.

-Petent No. 2,160,252.

JULIAN R.

May 50, 1959.

SGHLAEGER It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows: Page 5, first column, line 1 9, for "himidity" read lmmidity;line 71 for the word "herematter" reed hereinberore; page 7 firstcolumn, line 1 8, after "crystal" insert a. period,- and second column,lines 2, 7, and 11, claims 5, i and 5 respectively, for the claimreference numeral "23" read 2; line 35, claim 10, for 0'." read 160? 0.;and that the said Letters Patent should be read with this correctiontherein that the same may conform to the record of the case in thePatent Office.

'Signed and sealed this 15th day of August, A. D. 1939.

(Seal) Leslie Frazer licting Commissioner of Patents.

